This invention relates to a method of growing dielectric optical waveguides and more particularly to optical waveguides of arcuate structure grown preferentially by selective liquid phase epitaxy.
As improved sources and associated devices have been and continue to be developed, the feasibility of optical data processing systems is becoming an acceptable premise. In this connection, the integration of optical devices in a manner similar to the well-known integrated circuit may enable multiple data processing functions to be performed on a small area of material, provided the necessary miniaturization of the optical components and integration of the separate functions can be achieved. In general, integrated optics would include, for example, a source for light generation, propagation, modulation and directional coupling, wherein all of these functions are integrated so as to take place on a single substrate. Integrated optics as herein described is more fully disclosed in "Integrated Optics," Esther M. Conwell, Physics Today, May, 1976.
A key element in the integrated optical circuit is the optical waveguide that confines the propagation of light not only in straight lines, but also around bends, and at relatively low losses. In integrated optical circuits requiring complex processing of information, it will be necessary to utilize bends, curves, and dividers in the waveguide section. The previously known waveguides are based on having a region of higher index of refraction surrounded by lower effecting index media to confine and propagate the light between active components of the integrated optical circuit. For slab waveguides this has been accomplished either by changing the carrier concentration or type or by changing the composition of the layers. Straight channel waveguides have been fabricated by a number of different techniques to confine and propagate the light between active components of the integrated optical circuit. These techniques include electro-optical stripelines, diffusion, proton bombardment, metal gap guides, and various etched techniques such as optical stripelines, rib waveguides, and etched channel waveguides.
The lowest loss optical waveguides are those which have the smoothest confining interfaces and the smallest refractive index steps. Thus, a deeply etched channel waveguide consequently having roughly etched sidewalls causes radiation to be scattered into untrapped modes. The ribbed waveguide and the dielectric strip waveguide both involve a structure in which the lateral change in the index of refraction is very small. When these waveguides are formed into arcuate structures, this index of refraction discontinuity in the lateral direction causes the light confined around a bend structure to be very poor.
Although low loss straight channel waveguides have been described in literature using a number of these techniques, there is little experience with bends in going around corners or in other arcuate structures. It was previously believed that any epitaxial growth of arcuate semiconductor structures would result in a geometric structure having sharply faceted sidewalls which would scatter the light being propagated therethrough into untrapped modes. Calculations based on Butler's waveguide theories and experimental data (presented at the Device Research Conference at Salt Lake City in June of 1976; submitted for publication in Journal of Applied Physics in September of 1976) indicate that for thin dielectric stripeline waveguides with a large discontinuity in the index of refraction, transmission of only 60% can be obtained for a radius of curvature of 125 mils. Such losses are impractical for complex processing in integrated optical circuits, and larger radii which will provide for lower losses are also impractical due to the greater chip surface area they would occupy.
Dielectric optical waveguides and a method for fabricating the same by vapor phase epitaxy for straight configurations have been taught to us by D. W. Shaw, copending U.S. patent application Ser. No. 458,628, filed Apr. 8, 1974, assigned to Texas Instruments Incorporated, the same assignee of this patent application. In accordance with the teachings of Shaw, the straight optical waveguides comprise layers of semiconductor material of the same conductivity type, wherein one of the semiconductor layers has a relatively high refractive index, while the other semiconductor layers have a relatively low refractive index with the light being propagated through the semiconductor layer having the relatively higher refractive index. The substrate is a semiconductor material crystallographically oriented to expose a surface parallel to a relatively fast growing plane or subsequent vapor phase epitaxial growth.
The dielectric mask opening is oriented on the planar surface of the substrate material in such a manner that the sides and end walls of the opening in the dielectric mask will be oriented to lie in planes parallel to the slow growing crystallographic planes of the semiconductor substrate. The preferred orientation of the dielectric mask opening to the crystallographic planes of the substrate allows for the formation of an optical waveguide wherein each of its four planar surfaces including top, bottom, and side surfaces are formed as smooth planar faceted growth surfaces.